Research studies on adult and embryonic stem cells (ESCs) have undergone significant advances during the past few years. Experimental evidence shows that both stem cells could have future benefits in the area of regenerative medicine. Adult stem cells have been shown to have potential benefits for such diseases or conditions as diabetes mellitus, liver disease, cardiac dysfunction, Alzheimer's disease, Parkinson's disease, spinal cord injuries, bone defects, and genetic abnormalities (Herzog, et al. (2003) Blood 102:3483-3493; Petersen (2001) Blood Cell. Mol. Dis. 27:590-600; Ikehara (2003) Bone Marrow Transpl. 32:S73-S75; Nishimura, et al. Stem Cells 21:171-180; Hescheler & Fleischmann (2001) J. Clin. Invest. 108:363-364; Orlic, et al. (2001) Nature 410:701-705; Kehat, et al. (2001) J. Clin. Invest. 108:407-414). Furthermore, adult stem cells have been extensively studied for repair of orthopedic conditions, such as bone, cartilage, and tendon defects (Luyten (2004) Curr. Opin. Rheumatol. 16:599-603; Montufar-Solis, et al. (2004) Ann. Biomed. Eng. 32:504-509).
ESCs are considered the prototype stem cells due to their innate ability to differentiate into all possible cells, and are therefore invaluable sources for tissue repair (Hescheler & Fleischmann (2001) supra). Despite the remarkable potential of ESC in medicine, the obvious limit is the need for a safe match with respect to the Major Histocompatibility Complex Class II (Cogle, et al. (2003) Mayo Clin. Proc. 78:993-1003). ESCs could become functionally unstable when placed in an in vivo microenvironment and develop into tumors (Takahashi, et al. (2003) Nature 423:541-545). Another important consideration for ESCs is the stage of development, since these cells might be in transit to committed cells and would therefore express various developmental genes. This molecular change might not be evident since the ESC might not show phenotypic changes. Thus, it is conceivable that cells generated from ESCs could be dysfunctional if the originating stem cells are already committed to form cells of another tissue. Given these arguments, clinical application of ESCs would require robust examination of gene expressions prior to the generation of different cell types.
Clinical application of hematopoietic stem cells (HSCs) is controversial. Some reports show evidence of transdifferentiation by HSC (Orlic, et al. (2001) Proc. Natl. Acad. Sci. USA 98:10344-10349). Others report that transdifferentiation of HSC could be mistaken by cell fusion between HSC and cells of other tissues (Sapienza (2002) Proc. Natl. Acad. Sci. USA 99:10243-10245). An issue that is overlooked with respect to HSCs is the low efficiency that one could achieve in their expansion by current in vitro methods. Thus, to acquire sufficient HSCs for clinical use would require invasive procedures upon the donors. Another issue with HSC is that a population of HSC that is deemed stem cells by phenotypic analyses is generally heterogeneous, and could include cells that are committed towards a particular lineage (Shi, et al. (2004) Blood 104:290-294). Future application of HSC in repair medicine requires further research with the appropriate cell subset. Given an ideal situation where the candidate HSC is identified, there are ethical issues on the amount of bone marrow aspirates that should be taken from a donor.
Accordingly, alternative use of adult stem cells has been examined (Castro, et al. (2002) Science 1297:1299). Mesenchymal stem cells (MSC) show promise among the adult stem cells. MSCs are major adult bone marrow stem cells with multilineage potential (Morrison, et al. (1995) Annu. Rev. Cell Dev. Biol. 11:35-71; Deans & Moseley (2002) Exp. Hematol. 28:875-884). MSC could be used across allogeneic barriers due to their unique immune property (Potian, et al. (2003) J. Immunol. 171:3426-3434). This property of MSC is demonstrated by the ability to facilitate bone marrow transplantation (Koc, et al. (2002) Bone Marrow Transpl. 30:215-222; Rinder, et al. (2003) Exp. Hematol. 31:413-420; Prockop, et al. (2003) Proc. Natl. Acad. Sci. USA 100:11917-11923; Proia & Wu (2004) J. Clin. Invest. 113:1108-1110; Angelopoulou, et al. (2003) Exp. Hematol. 31:413-420). Furthermore, MSC are easily obtained from adult bone marrow, and can be expanded by simple in vitro procedures (Bianco, et al. (2001) Stem Cells 19:180-192). MSCs have been shown to transdifferentiate into cells of other germ layers (Bianco & Robey (2001) Nature 414:118-121; Munoz-Elias, et al. (2004) J. Neurosci. 24:4585-4595).
The generation of MSCs into neurons has been studied (Hofstetter, et al. (2002) Proc. Natl. Acad. Sci. USA 99:2199-2204; Qian & Saltzman (2004) Biomaterials 25:1331-7; Jiang, et al. (2002) Nature 418:41-49). However, for the most part, these studies characterized the transdifferentiation of MSC based on morphology, phenotypic changes and action potential (Hofstetter, et al. (2002) supra; Qian & Saltzman (2004) supra; Jiang, et al. (2002) supra; Dezawa, et al. (2004) J. Clin. Invest. 113;1701-1710). Synaptic transmission has not been reported for neurons derived from MSC. Cells similar to MSCs have been shown to survive in the brain (Bianco & Robey (2001) supra).
Needed in the art is a method for generating functional neurons from adult human MSC. The present invention meets this long-felt need.